Environmental Engineering Reference
In-Depth Information
3 million liters of Caspian ballast water, containing a diversity of plants and animals
(even t he cholera bacterium Vibrio cholerae has been found in ballast water). One
management solution is to make it compulsory (rather than voluntary) to dump
ballast water in the open ocean - this is now the case for the Great Lakes. Other
possible methods involve fi lter systems when loading ballast water, and on-board
treatment by ultraviolet irradiation or waste heat from the ship's engines.
The most damaging invaders are not simply those that arrive in a new part of the
world - the subsequent pattern and speed of their spread also matters. Zebra mussels
( Dreissena polymorpha ) have had a devastating effect (Section 1.2.5) since arriving
in North America via the Caspian Sea/Great Lakes trade route. The expansion of
their range occurred quickly through all commercially navigable waters, but over-
land dispersal into inland lakes, mainly attached to recreational boats, has been
much slower (Kraft & Johnson, 2000). Geographers have developed a method to
predict human dispersal patterns based on distance to and attractiveness of destina-
tion points, and Bossenbroek et al. (2001) adopted their modeling approach to
predict the spread of zebra mussels through the inland lakes of Illinois, Indiana,
Michigan and Wisconsin (364 counties in all). The model, which we need not con-
sider in detail, has three steps dealing respectively with the probability of a boat
traveling to a zebra mussel source and inadvertently picking up mussels, the prob-
ability of the same boat making a subsequent outing to an uncolonized lake, and
the probability of zebra mussels becoming established in the uncolonized lake.
Figure 4.7a shows the number of lakes colonized by zebra mussels in each county
as predicted by the model. This can be a fraction of one, because of the probabilistic
(a)
(b)
N
N
1
1
1
1
1
1
1
7
1
3
1
1
1
2
3
1
1
Infected lakes
0-0.25
0.25 - 0.5
0.5 -1
1-3
>4
3
4
1
4
1
5
100 km
100 km
Fig. 4.7 (a) The predicted distribution (based on 2000 iterations of a computer model of dispersal) of inland lakes
colonized by zebra mussels in 364 counties in the USA; the large lake in the middle is Lake Michigan, one of the Great
Lakes of North America. (b) The actual distribution of colonized lakes as of 1997. The numbers refer to the number of lakes
in each county predicted to be invaded (a) or actually invaded (b). (After Bossenbroek et al., 2001.)
 
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